1. Field of the Invention
The invention relates to a composite coating. Specifically, the invention relates to a composite coating capable of use as a lubrication coating at high temperatures.
2. Background of the Disclosure
There is an increasing need for durable lubricant materials that operate over a wide range of temperatures and at high speeds and/or for long durations. Such materials find increasing need in space satellites and vehicles, adiabatic diesel and advanced turbo machinery, process control valve stems, dry running stirling engine cylinders, high speed foil air bearings, rotating face valves, butterfly valve stems, and the like. Numerous advances in the art have been made over the last thirty years since early self-lubricating compositions, such as composites of silver, platinum, molybdenum disulfide, lead oxide and silicon dioxide were disclosed in U.S. Pat. No. 3,199,934. A significant advance was made by Sliney as a porous nickel-chromium alloy in which is dispersed, via infiltration, a metal fluoride eutectic and, optionally, a coating of the eutectic and silver on the outer bearing surface as disclosed in U.S. Pat. No. 3,419,363. However, the porous metal provides a greatly increased surface area and high temperature oxidation of these porous sintered metals posed significant problems at temperatures above about 700 degrees Centigrade. This led to the development of another self-lubricating composite in which the metal component is a porous high temperature alloy body which is either infiltrated with both metal fluorides and glass or via plasma spray co-deposition of the component powders, as disclosed in U.S. Pat. No. 4,214,905. The presence of the glass in the composite increased the oxidation resistance of the metal binder. The silver is electrodeposited on the metal. A still further improvement in the art was the development of a more wear resistant composite of nickel-cobalt bonded chromium carbide with metal fluoride and silver which is known as PS/PM200 and is disclosed in U.S. Pat. No. 4,728,448 and which has been extensively published in the literature. While this material has met with much success as a durable, long lasting, wear resistant self-lubricating composite useful over a wide temperature range, it is expensive and the chromium carbide component is so hard as to require costly diamond grinding to achieve the desired dimensions prior to service. Further, at very high temperatures of 800 degrees Centigrade or more in an oxidative environment such as air, the chromium carbide tends to oxidize. This degrades the friction and wear properties and causes slight dimensional swelling of the composite body. It would be a significant improvement to the art if a material were available with the strength, low friction and wear characteristics of the PS/PM200, without the drawbacks of very high temperature oxidative instability, high component cost and the need for expensive diamond grinding to polish the bearing surfaces and achieve the proper dimensions of the composite body.
Another lubricant was developed by the assignee of the present invention—PS300, containing a nickel chrome matrix with chrome oxide hardeners combined with silver and fluoride solid lubricants. Embodiments of the PS300 lubricant are disclosed in U.S. Pat. No. 5,866,518. While the PS300 coating has proven successful in many applications including foil bearings, opportunities for improvements continue to exist. For example, a thermal expansion mismatch between PS300 and superalloy substrates may cause thermal cycle fatigue spalling in repeated use at temperatures above 500 degrees Centigrade. The PS300 composition was tailored to modify the expansion properties without degrading the coatings tribiological performance. PS304, which is described in the '518 patent, was selected as the preferred coating for deposition on superalloys.
However, several shortcomings of the PS304 were identified through oil-free gas turbine engine testing. For example, coating dimensional stability is an intrinsic weakness of PS304. Early foil bearing and oil-free engine tests conducted at high temperature showed that the coating thickness increased significantly, as much at 7 percent, when the coating is exposed to air temperatures over 500 degrees Centigrade. Coating cohesive strength and hardness also increased. After extensive study, it was determined that chromium oxide phase precipitates were formed inside the matrix phase of the coating resulting in a volume increase or swelling action. The lack of dimensional stability has been overcome by including a high-temperature, extended time (e.g. 150 hours) heat treatment in air prior to final grinding of the coating. While the resulting coating exhibited dimensional stability adequate for many applications, the added heat treatment step added to the manufacturing cost and complexity limiting applications. Further, a significant concern is that long-term oxidation, for example after 10,000 hours, might degrade the coating integrity.
Yet another problem with the PS304, especially for use with foil bearings, is its initially high surface roughness, caused by its porosity, coarse microstructure and morphology, which may result in reduced foil gas bearing load capacity. Following finishing by grinding, PS304 exhibits a typical surface roughness of about 0.8 micrometers root-mean-square (“rms”). This level of roughness is significantly higher than the industry standard for shaft coatings such as thin dense chrome that typically has a smoother finish on the order of 0.2 micrometers rms. This phenomenon diminishes after the bearings are “broken in” through cyclic sliding cycles at high temperatures but the reduced “as installed” load capacity can preclude the use of PS304 in certain applications. To overcome this shortfall, a research effort identified that burnishing the ground, rough and porous surface with a sacrificial, temporary lubricant, like graphite or molybdenum disulfide, restored bearing load capacity until the breaking in the process occurred.
Therefore, opportunities exist to improve the PS300 series of coatings to overcome known problems and also to develop solutions that may achieve comparable performance but at lower cost and manufacturing complexity.